Solar ponds employing an increasing salt density layer as a function of depth are known. These ponds ideally employ three zones, an upper convective zone, a middle non-convecting zone, and a lower convective zone. The concentration of salt is lowest at the top region and highest in the lower region. In the middle non-convecting region, the salt concentration increases with depth. The middle zone allows transmission of a portion of solar energy into the lower zone while insulating from the upward conduction of thermal energy as a result of the increasing salt density.
The salt gradient solar pond appears to be cost effective even though its efficiency is very low, i.e., 2% energy recovery overall, because the plant capital costs, maintenance and operating cost forecasts are modest. At this time, it is not known for certain whether the existing pilot projects will be able to be scaled up in size successfully.
There are at least two serious technical concerns in contemplating scaling up a pond facility. The first most serious concern is with the Rankine cycle heat engine operation. In view of the fact that to date the temperatures in the pond lower level have only been able to reach 90.degree. C., the Rankine cycle efficiency is very low, on the order of 8%. Since the Rankine cycle efficiency curve is quite steep in this vicinity, very small temperature changes can cause serious further reduction in efficiency. Temperature losses in the ducts to the engine and/or periods of overcast could lower the pond temperature and because of the sharp slope of the efficiency curve as a function of temperature, the efficiency would be seriously reduced, increasing the cost/watt of the project to a non-breakeven value. A second major problem relates to maintaining the integrity of the brine zones. Water movements such as surface wave action and the pumping of water to the heat exchanger can cause disruption currents causing the salt gradient to be more uniform resulting in upward thermal convection.